1. Field of the Invention
The invention relates to inhibiting amyloid deposit formation and dissolving pre-formed amyloid deposits and to methods and pharmaceutical compositions for treating brain inflammation and for treating inflammation associated with amyloid or amyloid-like deposits within the brain and elsewhere in the body.
2. Description of the Related Art
Plaque forming diseases are characterized by the presence of amyloid plaques deposits in the brain as well as neuronal degeneration. Amyloid deposits are formed by peptide aggregated into an insoluble mass. The nature of the peptide varies in different diseases but in most cases, the aggregate has a beta-pleated sheet structure and stains with Congo Red dye. In addition to Alzheimer's disease (AD), which includes early onset Alzheimer's disease, late onset Alzheimer's disease, and presymptomatic Alzheimer's disease, other diseases characterized by amyloid deposits are, for example, SAA amyloidosis, hereditary Icelandic syndrome, multiple myeloma, and prion diseases. The most common prion diseases in animals are scrapie of sheep and goats and bovine spongiform encephalopathy (BSE) of cattle (Wilesmith and Wells, 1991). Four prion diseases have been identified in humans: (i) kuru, (ii) Creutzfeldt-Jakob Disease (CJD), (iii) Gerstmann-Streussler-Sheinker Disease (GSS), and (iv) fatal familial insomnia (FFI) (Gajdusek, 1977; and Tritschler et al. 1992).
Prion diseases involve conversion of the normal cellular prion protein (PrPC) into the corresponding scrapie isoform (PrPSc). Spectroscopic measurements demonstrate that the conversion of PrPC into the scrapie isoform (PrPSc) involves a major conformational transition, implying that prion diseases, like other amyloidogenic diseases, are disorders of protein conformation. The transition from PrPC to PrPSc is accompanied by a decrease in α-helical secondary structure (from 42% to 30%) and a remarkable increase in β-sheet content (from 3% to 43%) (Caughey et al, 1991; and Pan et al, 1993). This rearrangement is associated with abnormal physiochemical properties, including insolubility in non-denaturing detergents and partial resistance to proteolysis. Previous studies have shown that a synthetic peptide homologous with residues 106-126 of human PrP (PrP106-126) exhibits some of the pathogenic and physicochemical properties of PrPSc (Selvaggini et al, 1993; Tagliavini et al, 1993; and Forloni et al, 1993). The peptide shows a remarkable conformational polymorphism, acquiring different secondary structures in various environments (De Gioia et al, 1994). It tends to adopt a β-sheet conformation in buffered solutions, and aggregates into amyloid fibrils that are partly resistant to digestion with protease. Recently, the X-ray crystallographic studies of a complex of antibody 3F4 and its peptide epitope (PrP 104-113) provided a structural view of this flexible region that is thought to be a component of the conformational rearrangement essential to the development of prion disease (Kanyo et al, 1999). The identification of classes of sequences that participate in folding-unfolding and/or solubilization-aggregation processes may open new direction for the treatment of plaque forming disease, based on the prevention of aggregation and/or the induction of disaggregation (Silen and Agard, 1989; Frenkel et al, 1998; Horiuchi and Caughey, 1999).
Alzheimer's disease (AD) is a progressive disease resulting in senile dementia. Broadly speaking, the disease falls into two categories: late onset, which occurs in old age (typically above 65 years) and early onset, which develops well before the senile period, e.g., between 35 and 60 years. In both types of the disease, the pathology is similar, but the abnormalities tend to be more severe and widespread in cases beginning at an earlier age. The disease is characterized by two types of lesions in the brain, senile plaques and neurofibrillary tangles. Senile plaques are areas of disorganized neutrophils up to 150 mm across with extracellular amyloid deposits at the center, visible by microscopic analysis of sections of brain tissue. Neurofibrillary tangles are intracellular deposits of tau protein consisting of two filaments twisted about each other in pairs.
The principal constituent of the senile plaques is a peptide termed amyloid beta (Aβ) or beta-amyloid peptide (βAP or) βA). The amyloid beta peptide is an internal fragment of 39-43 amino acids of a precursor protein termed amyloid precursor protein (APP). Several mutations within the APP protein have been correlated with the presence of Alzheimer's disease (Goate et al, (1991), valine717 to isoleucine; Chartier Harlan et al, (1991), valine717 to glycine; Murrell et al, (1991), valine717 to phenylalanine; Mullan et al, (1992), a double mutation, changing lysine595-methionine596 to asparagine595-leucine596).
Such mutations are thought to cause Alzheimer's disease by increased or altered processing of APP to beta-amyloid, particularly processing of APP to increased amounts of the long form of beta-amyloid (i.e., Aβ1-42 and Aβ1-43). Mutations in other genes, such as the presenilin genes, PS1 and PS2, are thought indirectly to affect processing of APP to generate increased amounts of long form beta-amyloid (see Hardy, TINS 20, 154, 1997). These observations indicate that beta-amyloid, and particularly its long form, is a causative element in Alzheimer's disease.
Other peptides or proteins with evidence of self aggregation are also known, such as, but not limited to, amylin (Young et al, 1994); bombesin, cerulein, cholecystokinin octapeptide, eledoisin, gastrin-related pentapeptide, gastrin tetrapeptide, somatostatin (reduced), substance P; and peptide, luteinizing hormone releasing hormone, somatostatin N-Tyr (Banks and Kastin, 1992).
Publications on amyloid fibers indicate that cylindrical β-sheets are the only structures consistent with some of the x-ray and electron microscope data, and fibers of Alzheimer Aβ fragments and variants are probably made of either two or three concentric cylindrical β-sheets (Perutz et al., 2002). The complete Aβ peptide contains 42 residues, just the right number to nucleate a cylindrical shell; this finding and the many possible strong electrostatic interactions in β-sheets made of the Aβ peptide in the absence of prolines account for the propensity of the Aβ peptide to form the extracellular amyloid plaques found in Alzheimer patients. If this interpretation is correct, amyloid consists of narrow tubes (nanotubes) with a central water-filled cavity. Reversibility of amyloid plaque growth in-vitro suggests steady-state equilibrium between βA in plaques and in solution (Maggio and Mantyh, 1996). The dependence of βA polymerization on peptide-peptide interactions to form a β-pleated sheet fibril, and the stimulatory influence of other proteins on the reaction, suggest that amyloid formation may be subject to modulation. Many attempts have been made to find substances able to interfere with amyloid formation. Among the most investigated compounds are antibodies, peptide composed of beta-breaker amino acids like proline, addition of charged groups to the recognition motif and the use of N-methylated amino-acid as building blocks (reviewed by Gazit, 2002).
Cyclic peptides made of alternate D and L residues form such nanotubes that kill bacteria by inserting themselves into membranes and depolarizing them (Perutz et al., 2002). There is some suggestion that some amyloid fibers might be conductors and kill cells by the same mechanism.
Aromatic compounds such as congo red that can insert themselves into gaps between helical turns might destabilize the cylindrical shells and initiate this process, but prevention would be more effective and probably easier to achieve (Perutz et al., 2002).
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